This invention relates to the fabrication of ferrite bodies, and in particular to the use of proper binder systems for achieving high permeability and/or fast thruputs in production.
Ferrites are generally defined as compounds of ferric oxide, Fe.sub.2 O.sub.3, and a metallic oxide, MeO, where Me can be iron, manganese, zinc, cobalt, nickel, cadmium, magnesium or mixtures thereof. Such materials enjoy a wide use, for example, as magnetic cores in loading coils, choke coils and transformers, and as the gyromagnetic material in microwave circulators and isolators.
In the fabrication of ferrite materials a "binder system" is generally employed. This system usually includes (1) a dispersant to improve the solids to liquid ratio and facilitate milling, (2) an organic binder to impart green strength to the part fabricated by a suitable means such as pressing or slip casting, (3) a plasticizer to improve toughness and flexibility of the binder film and (4) a lubricant to minimize particle-to-particle friction and die wall friction during pressing.
The choice of a binder system will affect many important characteristics of the final product. For example, it is known that permeability is related to the grain size of the ferrite, although other factors such as zinc loss play a role. The choice of a binder to a large extent determines whether the proper grain size is achieved in a sufficiently short time to attain high permeability. Utilizing a typical prior art binder system comprising gum arabic, zinc stearate and polyvinyl alcohol, the maximum permeability generally obtained was 18,000. It is desirable to produce permeabilities of at least 20,000 for certain applications.
It is also desirable, for fast thruputs in production, to achieve a short firing cycle in the sintering of the material. The binder system will also have an effect on such firing times. For example, with the prior art binder system previously described, a firing cycle of 12 hours which includes a 6 hour soak time at 1345 degrees C. was typically required to produce permeabilities in the range of 4,000-8,000 in a standard 26/16 IEC type pot core. For economy in manufacture, a considerable shortening of the firing time is desirable.
It is therefore an object of the invention to provide a method for achieving high permeability ferrite bodies. It is a further object of the invention to produce medium range permeability ferrite bodies with a shortened firing cycle.